Here are the most important things to keep in mind if you want blazing high speed on your downhills.
Picking the fastest lane is the best bet to get ahead in a race and looking as far down the trail as possible is the only way to do it. Instead of staring at the trail in front of you, look as far ahead as possible so you can see obstacles with enough time to make the best response or detour. When learning to drive, you're told not to stare at anything for more than a second or two; this is a good tip for the trails as well. It's okay to let your eyes flip up and down the trail, but concentrate on looking ahead as much as possible.
Tune Your Bike
The fit, from frame to stem to pedals to bars to brake levers, is critical in your ability to go fast. If you're not comfortable on your bike, you won't be able to go fast. Tune your bike to your preference and size, and make sure your brakes and shifters are dead on. If you don't have confidence in your equipment, you can't go fast. Experiment with tire pressure and settings if you have suspension. Once your bike is dialed and race tuned, keep it that way even if you don't race. A clean, well lubricated and adjusted bike is a happy and fast bike. This will also make your bike last longer.
Practice makes perfect. It's extremely important to be able to predict how your bike will respond to a given situation and speed. Particularly in corners, you need to exactly how fast you and your bike can handle them. Learn what types of terrain are the fastest -- ride on hard packed terrain instead of much slower soupy sand.
Get In Shape
Get fit. You can't be a good downhiller unless you can pedal. Gravity is your friend, but it's everyone else's friend too. A big chainring will only give you more speed if you can use it.
Learning to hop your bike over obstacles is extremely important, not to mention fun. "Bunnyhopping" is a downward compression followed by an upward spring. It is a must have skill for descending quickly, and usually you don't need much height on you jumps, especially at high speed. Having your wheels off the ground slows you down though, so time your jumps so you can pedal as much as possible. Learn to do it several times in rapid succession. Learn to do it and be able to change direction slightly while in the middle of a jump especially in really rough, rocky or rutted sections.
Ride the Corners
Mountain bike tires are designed to dig in at their edges, but be careful -- sometimes (sand) they dig in just a bit too much. Keep your center of gravity low by putting as much weight as possible on the outer pedal. The sharper the turn the more weight you should move from your inner to your outer pedal. Remember to put some weight on the handle bars for more control over steering. To go even faster around corners safely, keep your inside foot loose and ready to put down if the wheel should slip. If you don't have suspension don't worry about the wheels wanting to bounce around. Let them. Just keep your weight centered, look where you want to go and keep the bars pointed in that general direction. In races, make note of the guy in front of you, if he drives off a cliff make sure you are ready for it.
Even Speed Demons Have to Slow Down
Learn when to go slow. It's a lot faster to slow down and not fall off then to encounter an obstacle at an unmanageable speed. Trust me, if you go too fast and smear your brains on a tree, it's a whole lot slower then just hitting the brake. If brakes weren't there to be used, they wouldn't be there at all. The front wheel provides most of your braking ability, but makes steering on corners pretty squirrelly.
On very steep descents it is important to use your front brake, as it supplies most of your braking power. However, when there is a curve in the hill you will want to let off on the front brake in order to regain steering power. To make sure you don't go over the handle bars you have to lean back. The best way is to remain low, but take your butt off the seat and move it back, sometimes even right behind the seat. When leaning back, be careful that you do not loosen your grip on the handle bars. Leaning back also has the effect of giving your rear wheel more traction and braking power.
Use Your Helmet
Wearing your helmet gives you the reassurance that slamming into a tree won't end your cycling career so you can push it to the limit confidently and fearlessly. Glasses, gloves, cycling shoes and shorts. A bug or a bit of mud in your eye can cost you those precious seconds in downhill racing. A spill without a helmet and gloves is guaranteed to make a painful mess.